Greek letter often used in mathematics and physics, and other branch of science. Some Internet subculture adopted it to make their own identities. If you are asking about “sigma male” it is pseudo identity where someone who identifies as a man is like “I don’t care about others opinions and I will live however I want” it is good as a joke, but many many almost majority (or loud minority idk, I’m not into that anymore) are bigoted towards anyone different and use “being sigma male” as an excuse to that. It is useless in real world because you don’t need separate identity for personality type, just live. Ironic that people who unironically group themselves and others into that whole “alpha/beta/sigma” groups are the ones who say gender is not construct (gender=sex) while their whole categorizations of assigning greek letters to personalities are social constructs.